The value of tcp_max_syn_backlog is not a single, fixed number but rather a dynamic parameter that varies depending on the system's memory and configuration. It represents the maximal number of connection requests that the kernel will remember (SYN packets received) for which it has not yet received an acknowledgment (ACK) from the connecting client.
Understanding tcp_max_syn_backlog
This kernel parameter plays a crucial role in managing incoming TCP connections, particularly during high traffic or potential denial-of-service attacks like SYN floods. When a client initiates a connection, it sends a SYN packet. The server responds with a SYN-ACK, and then waits for the client's final ACK. During this waiting period, the connection is in a "half-open" state. tcp_max_syn_backlog determines the maximum number of such half-open connections that the system will queue.
How the Value is Determined
While there isn't one universal exact value, its determination follows a specific rule:
- Minimum Value: The minimal value for
tcp_max_syn_backlogis 128. This is typically the default for systems with low memory. - Dynamic Scaling: For systems with more memory, this value will automatically increase in proportion to the available memory of the machine. This allows systems with greater resources to handle more concurrent half-open connections efficiently.
Therefore, while 128 is the base minimum, the actual default value on a modern server with ample memory will likely be significantly higher, often 1024, 2048, or even 4096, depending on the kernel version and memory.
Importance and Practical Implications
Effectively managing tcp_max_syn_backlog is vital for server performance and security:
- Preventing SYN Flood Attacks: A SYN flood attack aims to exhaust the server's half-open connection queue by sending many SYN packets without completing the handshake. If
tcp_max_syn_backlogis too low, legitimate connection requests can be dropped during such an attack or even during legitimate high traffic. - Server Responsiveness: A sufficiently high value ensures that legitimate clients can establish connections even when the server is under moderate to heavy load, preventing connection timeouts and improving user experience.
- Memory Usage: While increasing the value helps with high load, it also means more memory is reserved for handling half-open connections. An excessively high value on a memory-constrained system could lead to other performance issues.
Checking and Adjusting the Value
On Linux systems, tcp_max_syn_backlog is a sysctl parameter.
1. Checking the Current Value
You can easily check the current tcp_max_syn_backlog value using the sysctl command:
sysctl net.ipv4.tcp_max_syn_backlogThis will output something like net.ipv4.tcp_max_syn_backlog = 1024 (the number will vary based on your system).
2. Temporarily Changing the Value
To increase the value temporarily (until the next reboot), you can use:
sudo sysctl -w net.ipv4.tcp_max_syn_backlog=2048Replace 2048 with your desired value.
3. Persistently Changing the Value
For changes to persist across reboots, you need to edit the /etc/sysctl.conf file or add a new configuration file in /etc/sysctl.d/.
- Edit
/etc/sysctl.conf:
Add or modify the following line:net.ipv4.tcp_max_syn_backlog = 2048 - Apply Changes:
After saving the file, apply the changes immediately without rebooting:sudo sysctl -p
Table: tcp_max_syn_backlog Management Commands
| Action | Command | Effect |
|---|---|---|
| Check current value | sysctl net.ipv4.tcp_max_syn_backlog |
Displays current setting |
| Set temporarily | sudo sysctl -w net.ipv4.tcp_max_syn_backlog=VALUE |
Applies until reboot |
| Set persistently | Edit /etc/sysctl.conf or /etc/sysctl.d/ |
Applies after reboot |
| Apply persistent changes | sudo sysctl -p |
Loads configuration |
Best Practices
- Monitor System Load: Observe your server's connection rates and
netstatoutput (e.g.,netstat -ntp | grep SYN_RECV) to understand current demands. - Balance Performance and Security: While a higher
tcp_max_syn_backlogcan improve performance under high load, be mindful of the memory implications. Also, it's just one part of a comprehensive SYN flood defense strategy. - Avoid Arbitrary Values: Base your adjustments on observed system behavior and specific application requirements rather than simply setting a very high number.
